In recent years, circuits for medium-voltage variable frequency drive (VFD) applications have received attention. Several novel methods have been introduced in the past decade. For example, in a circuit comprising series-connected inverters as described in U.S. Pat. No. 5,625,545 to Hammond, the disclosure of which is incorporated herein by reference in its entirety, an inverter or power cell 110 includes a three-phase diode-bridge rectifier 112, one or more direct current (DC) capacitors 114, and an H-bridge inverter 116. The rectifier 112 converts the input 118 alternating current (AC) voltage to a substantially constant DC voltage that is supported by the capacitors 114 that are connected across the rectifier 112 output. The output stage of the inverter 110 includes an H-bridge inverter 116 that includes two poles, a left pole and a right pole, each with two devices. The inverter 110 transforms the DC voltage across the DC capacitors 114 to an AC output 120 using pulse-width modulation (PWM) of the semiconductor devices in the H-bridge inverter 116.
A circuit including power cells such as 110 in FIG. 1, when connected to a load, such as a motor, can provide power from an input source to the motor when operating in the motoring mode. Such a power cell may sometimes be referred to as a unidirectional or two-quadrant (2Q) cell. This is because when the four quadrants of speed and torque are considered, referring to FIG. 2, the operating characteristics 210 of this cell are such that it operates in either the quadrant where both speed and torque are positive (first quadrant 201) or the quadrant where both speed and torque are negative (third quadrant 203).
However, when the motor speed needs to be reduced, power from the motor needs to be absorbed by the inverter. This mode of operation, when power must be absorbed by the inverter, is referred to as the regeneration mode. In such situations, regenerative or four-quadrant cells are required. An example of a regenerative cell is shown in U.S. Pat. No. 6,301,130 to Hammond. As shown in FIG. 3, a regenerative power cell 360 may include an active front end 362 that serves as a first converter that uses insulated gate bipolar transistors (IGBTs) Q5-Q10 or other switching devices controlled by PWM. The first converter 362 is electrically connected in parallel to a second converter 364 and to one or more DC link capacitors 366. Such a cell receives power from a transformer 346 and delivers it to other cells in the group and a load 349. Referring to FIG. 2, this cell permits operating characteristics 220 in all four quadrants 201-204, including the quadrant where both speed and torque are positive (first quadrant 201), the quadrant where torque is positive and speed is negative (second quadrant 202), the quadrant where both speed and torque are negative (third quadrant 203), and the quadrant where torque is negative and speed is positive (fourth quadrant 204).
In the prior art, motor systems included two-quadrant or four-quadrant cells. However, systems that are designed to accommodate one or the other are limited in applicability. The disclosure contained herein describes attempts to solve one or more of the problems described above.